While there isn't special form for tailcall, they are not implicit. If the recursive call does something with the result, it's not a tailcall. Unless you are being cute with closures, it's already explicit in the AST.

  --> return myself(...)       # tail call
  --> return var * myself(...) # not a tail call

But there is indeed something to be said about a construct that enforces the tail call guarantee. The point of SICP's scheme though is to keep the number of language constructs to a minimum for educational purposes.